The presence of a progesterone binding protein in uterine cytosol and the nuclear fraction has been demonstrated in rabbit, human, baboon, cat, and dog. There is a quantitative increase in progesterone binding protein after estrogen administration. Intrauterine concentrations of progesterone relate directly to biological events associated with hormonal functions such as nidation and maintenance of pregnancy. It is proposed to establish whether or not a relationship exists between the concentration of progesterone receptors and progestational response. Following either partial or complete purification of the cytosol progesterone binding protein, interactions of the cytosol receptor with chromatin prepared from target and non-target tissues will be investigated. Recently, it was observed that in some abnormal human uterine tissues a progesterone receptor was absent while in the same preparation a substantial amount of estradiol-17 beta binding protein was present. Continuation of this line of investigation may lead to the possible separation of abnormal uterine tissues into those which may respond to progesterone therapy and those which may receive no benefit. A study of the correlation of biological potency of synthetic progestational compounds with their binding affinity for uterine progesterone receptors will be conducted. A positive correlation obtained from this analysis would facilitate the synthesis and rapid in vitro evaluation of new progestins.